Terrain awareness system

ABSTRACT

A terrain awareness system (TAS) provides LOOK-AHEAD/LOOK-DOWN as well as LOOK-UP terrain advisory and warning indications to the pilot of an aircraft of a hazardous flight condition. The TAS includes an airport data base as well as terrain data base that is structured to provide various resolutions depending on the topography of the particular geographic area of interest. Navigational data from a satellite-based navigational system, such as a global positioning system (GPS), is used to provide a LOOK-AHEAD/LOOK-DOWN and LOOK-UP terrain advisory and terrain warning indications based upon the current position and projected flight path of the aircraft. Since the terrain advisory and the warning signals are a function of the flight path of the aircraft, nuisance warnings are minimized.

PRIORITY CLAIM

[0001] This application is a continuation of application Ser. No.08/509,660, filed Jul. 31, 1995, attorney docket number 543-94-001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates to a terrain awareness system (TAS)and more particularly to a system for alerting a pilot of an aircraft ofa dangerous flight condition which monitors the position as well as thetrajectory of an aircraft based upon a satellite-based navigationsystem, such as a global positioning system (GPS), to provide aLOOK-AHEAD/LOOK-DOWN as well as LOOK-UP terrain advisory and warningindications based upon stored terrain data which provides relativelylonger warning times than known ground proximity warning systems whileminimizing nuisance warnings.

[0004] 2. Description of the Prior Art

[0005] Various systems are known in the art that provide warnings andadvisory indications of hazardous flight conditions. Among such systemsre systems generally known as ground proximity warning systems (GPWS)which monitor the flight conditions of an aircraft and provide a warningif flight conditions are such that inadvertent contact with terrain isimminent. Among the flight conditions normally monitored by such systemsare radio altitude and rate, barometric altitude and rate, air speed,flap and gear positions. These parameters are monitored and an advisorysignal and/or warning signal is generated when the relationship betweenthe parameters is such that terrain impact is likely to occur. Typicalexamples of such systems are disclosed in U.S. Pat. Nos. 3,715,718;3,936,796; 3,958,218; 3,944,968; 3,947,808; 3,947,810; 3,934,221;3,958,219; 3,925,751; 3,934,222; 4,060,793; 4,030,065; 4,215,334; and4,319,218, all assigned to the same assignee as the assignee of thepresent invention and hereby incorporated by reference.

[0006] While the above-referenced systems do provide advisory andwarning signals in the event of proximity to terrain, the warningsgenerated by such systems are based solely upon flight conditions of theaircraft and do not utilize any navigational information. Consequently,the sensitivity of such systems must be adjusted to provide adequatewarnings when a hazardous flight condition exists without generatingfalse or spurious warnings. However, such an adjustment can result in acompromise that may still result in nuisance warnings over terrainunique to particular geographic areas and shorter than desired warningtimes in yet other geographic areas.

[0007] Several attempts have been made to improve upon such groundproximity warning systems utilizing ground-based navigationalinformation. For example, U.S. Pat. Nos. 4,567,483; 4,646,244;4,675,823; and 4,914,436 all disclose ground proximity warning systemswhich monitor the position of the aircraft relative to stored terraindata in order to provide modified ground proximity warnings. However,the utility of such systems is limited. For example, the systemsdisclosed in U.S. Pat. Nos. 4,567,483 and 4,914,436 disclose groundproximity warning systems which utilize navigational data to modifypredetermined warning envelopes surrounding certain particular airports.

[0008] U.S. Pat. Nos. 4,646,244 and 4,675,823 disclose terrain advisorysystems which utilize various ground-based navigational inputs andstored terrain data to provide various ground proximity warning systemsbased on the position of the aircraft. In order to conserve memory, theterrain data is modeled in various geometric shapes as a function of theelevation of the terrain within the area defined by the geometric shape.The warning signals are generated as a function of the position of theaircraft relative to the model. While such systems do provide adequateterrain warnings, such systems may cause nuisance warnings under certainconditions since the predicted trajectory of the aircraft is not takeninto consideration.

[0009] Another problem with such systems is that they are based uponground-based navigational systems, such as VOR/DME and LORAN. Suchground-based navigational systems are being replaced with relativelymore accurate satellite systems, such as the global positioning system(GPS). Moreover, such ground-based systems are only able to providetwo-dimensional position data. Thus, with such systems, the altitude ofthe aircraft must be supplied by an auxiliary device, such as analtimeter, which increases the complexity of the system as well as thecost.

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to solve variousproblems associated with the prior art.

[0011] It is yet another object of the present invention to provide aterrain awareness system which provides increased warning times to thepilot of an aircraft of a hazardous flight condition while minimizingnuisance warnings.

[0012] It is yet another object of the present invention to provide asystem which provides a LOOK-AHEAD/LOOK-DOWN as well as a LOOK-UPterrain advisory and warning indications to the pilot of an aircraft ofa hazardous flight condition based upon the predicted trajectory of theaircraft.

[0013] It is yet another object of the present invention to provide aterrain awareness system which utilizes inputs from a satellite-basednavigation system, such as the global positioning system (GPS).

[0014] Briefly, the present invention relates to a terrain awarenesssystem (TAS) which provides LOOK-AHEAD/LOOK-DOWN as well as LOOK-UPterrain advisory and warning indications to the pilot of an aircraft ofa hazardous flight condition. The TAS includes an airport data base aswell as a terrain data base that is structured to provide variousresolutions depending on the topography of the particular geographicarea of interest. Navigational data from a satellite-based navigationsystem, such as a global positioning system (GPS), is used to provide aLOOK-AHEAD/LOOK-DOWN and LOOK-UP terrain advisory and terrain warningindications based upon the current position and projected flight path ofthe aircraft. Since the terrain advisory and the warning signals are afunction of the flight path of the aircraft, nuisance warnings areminimized.

BRIEF DESCRIPTION OF THE DRAWING

[0015] These and other objects of the present invention will be readilyapparent with reference to the following specification and attacheddrawings, wherein:

[0016]FIGS. 1A and 1B represent a block diagram of a terrain awarenesssystem (TAS) in accordance with the present invention;

[0017]FIG. 2 is a diagram of the world broken down into various latitudesegments and longitude segments in accordance with the presentinvention;

[0018]FIG. 3 is a graphical representation of the various memory map andresolution sequencing in accordance with the present invention;

[0019]FIG. 4A is an exemplary representation of the digital header andsubsquare mask word for identifying geographical areas in accordancewith the present invention;

[0020]FIG. 4B is a terrain map alternative to FIG. 3;

[0021]FIG. 5 is a graphical illustration of the LOOK-AHEAD distanceassuming the aircraft turns at a 30° angle in accordance with thepresent invention;

[0022]FIG. 6 is a graphical representation of a ΔH terrain floorboundary which forms the bases for a terrain advisory signal and aterrain warning signal in accordance with the present invention;

[0023]FIG. 7 is a graphical illustration of a terrain advisory signalfor an aircraft relative to the terrain and an airport for a conditionwhen the aircraft flight path angle is less than a first predeterminedreference plane or datum in accordance with the present invention;

[0024]FIG. 8 is similar to FIG. 7 but for a condition when the aircraftflight path angle is greater than the first predetermined referenceplane;

[0025]FIG. 9 is a graphical illustration of a terrain warning signal foran aircraft relative to the terrain and an airport for a condition whenthe aircraft flight path angle is greater than a second reference planeor datum in accordance with the present invention;

[0026]FIG. 10 is similar to FIG. 9 but for when the flight path angle ofthe aircraft is less than the second reference plane or datum;

[0027]FIG. 11 is a graphical illustration of a cut-off angle correctionboundary for a level flight condition in accordance with the presentinvention;

[0028]FIG. 12 is similar to FIG. 11 but for a condition when the flightpath angle of the aircraft is greater than a predetermined referenceplane or datum;

[0029]FIG. 13 is similar to FIG. 11 but for a condition when the flightpath angle of the aircraft is less than a predetermined reference planeor datum and also illustrates a BETA sink rate enhancement boundary inaccordance with the present invention;

[0030]FIG. 14 is a graphical illustration of the terrain advisory andsuperimposed terrain warning signals for an aircraft relative to theterrain for a condition when the aircraft flight path angle is less thana first reference plane or datum;

[0031]FIG. 15 is similar to FIG. 14 but for a condition when the flightpath angle is between a first datum and a second datum;

[0032]FIG. 16 is similar to FIG. 14 but for a condition when the flightpath angle is less than the second datum;

[0033]FIG. 17 is a graphical illustration of LOOK-AHEAD/LOOK-DOWNterrain advisory and warning boundaries in accordance with the presentinvention for a condition when the aircraft is descending;

[0034]FIG. 18 is similar to FIG. 17 but for a condition when an aircraftis climbing;

[0035]FIG. 19 is a graphical illustration of LOOK-UP terrain advisoryand warning boundaries in accordance with the present invention;

[0036]FIG. 20 is a graphical illustration of an aircraft during apull-up maneuver;

[0037]FIG. 21 is an exemplary block diagram for a system for generatinga signal representative of the altitude loss due to pilot reaction timeALPT, as well as altitude loss due to a pull-up maneuver ALPU inaccordance with the present invention;

[0038]FIG. 22 is a graphical illustration of an alternative methodologyfor generating a cut-off altitude boundary in accordance with thepresent invention;

[0039]FIG. 23 is a block diagram of the display system in accordancewith the present invention;

[0040]FIG. 24 is a functional diagram of the display data format inaccordance with the ARINC 708/453 standard for a weather radar display;

[0041]FIG. 25 is a plan view of a background terrain displayillustrating the variable density dot pattern in accordance with thepresent invention;

[0042]FIG. 26 is similar to FIG. 25 but additionally illustrates theterrain threat indications;

[0043]FIG. 27 is a block diagram illustrating the configuration of aflash ROM for a terrain data base and a RAM used with the presentinvention;

[0044]FIG. 28 illustrates a wedding-cake configuration for a RAMutilized in the present invention;

[0045]FIG. 29 is a diagram of the file format of the files in theterrain data base in accordance with the present invention;

[0046]FIG. 30 illustrates a terrain map of the terrain data base inaccordance with the present invention illustrating the highest elevationin each of the cells within the map;

[0047]FIG. 31 is similar to FIG. 30 and illustrates a method forcorrecting various cells in the vicinity of a runway;

[0048]FIG. 32 is a simplified elevational view of certain dataillustrated in FIG. 31;

[0049]FIG. 33 is a diagram of a display illustrating the weather radarsweep spokes and the determination of the range increments;

[0050]FIG. 34 is a block diagram of the display system in accordancewith the present invention illustrating the memory configuration and thedata flow;

[0051]FIG. 34A is a block diagram of a configuration alternative to thatshown in FIG. 34;

[0052]FIG. 35 is a diagram of a terrain map illustrating thedetermination of the look-ahead vector array in accordance with thepresent invention;

[0053]FIG. 36 is a diagram of a display illustrating a radar sweep spokeand a method for determining the range increments of the radar sweepspoke;

[0054]FIG. 37 is a terrain map illustrating the computation of thehazardous terrain in accordance with the present invention;

[0055]FIG. 38 is a diagram of a terrain map superimposed on a displayscreen illustrating the sweep range and the determination of theincremental range increments;

[0056]FIG. 39 is a diagram of a pixel map of the display in accordancewith the present invention;

[0057]FIG. 40 is a diagram of the different fractal patterns used toprovide the variable density display of non-hazardous displayindications in accordance with the present invention;

[0058]FIG. 41 is a diagram illustrating the determination of the X and Yincrements of the display screen; and

[0059]FIG. 42 is a diagram of the method for superimposing the fractalpatterns on the displayed pixel map in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0060] Referring to FIG. 1, a terrain awareness system (TAS), generallyidentified with the reference number 20, is illustrated. As will bediscussed in more detail below, the TAS 20 utilizes inputs from asatellite-based navigational system, such as global positioning system(GPS) 22 (longitude, latitude, altitude, groundtrack, ground speed),and/or an FMS/IRS navigational system which may be updated by the GPSand/or DME/DME, a terrain data base 24, an airport data base 26 andcorrected barometric altitude, for example from a barometric altimeter28 to provide a LOOK-AHEAD warning system which provides relativelylonger warning times while minimizing nuisance warnings. Since the TAS20 does not require a radio altimeter input, the system can be used withcertain aircraft, such as commuter aircraft, which are not normallyequipped with a radio altimeter.

[0061] The current longitude and latitude of the aircraft from the GPS22 are applied to an Airport and Terrain Search Algorithm, indicated bya block 29, which includes location search logic for determining theterrain data, as well as the airport data surrounding the aircraft. Suchsearch logic is described in detail in U.S. Pat. Nos. 4,675,823 and4,914,436 assigned to the same assignee as the present invention andhereby incorporated by reference. The GPS inputs, along with terrain andairport data surrounding the aircraft from the search algorithm,indicated by the block 29, are applied to the LOOK-AHEAD warninggenerator 30, which provides both terrain advisory and terrain warningsignals based upon the position and projected flight path of theaircraft. The LOOK-AHEAD warning generator 30 may provide both an auralwarning by way of a voice warning generator 32 and speaker 34, and/or avisual warning by way of a map or a display 36, as discussed below.

Global Positioning System

[0062] The primary positional information for the aircraft is providedby the GPS 22, such as disclosed in U.S. Pat. Nos. 4,894,655; 4,903,212;4,912,645; 4,954,959; 5,155,688; 5,257,195; 5,265,025; 5,293,163;5,293,318; and 5,337,242, all hereby incorporated by reference. The GPS22 includes a GPS receiver 37 and a combination GPS receiver and monitor38. The positional inputs from the GPS 22 are processed by a deadreckoning algorithm, indicated by the block 40, for example as discussedin detail in U.S. Pat. No. 5,257,195, to compensate for temporaryoutages of the GPS 22, such as loss or masking of satellites. The deadreckoning algorithm can be replaced by an additional navigationalsystem, such as an FMS or an IRS, which can be used during GPS outages.

[0063] A differential GPS receiver 42 can also be used to provide anindication of a GPS altitude. As long as the differential GPSinformation is received and four more satellites are visible, thedifferential GPS information from the differential GPS receiver 42 issufficiently accurate for the TAS 20. If the differential GPS receiver42 is not available, a compound altitude signal may be generated by theblock 44 and connected to the LOOK-AHEAD warning generator 30 by way ofa single-pole, double-throw switch 46, under the control of a signal“DIFF GPS AVAILABLE”, which indicates when the differential GPS receiver42 is available. In a first mode when differential GPS information isavailable, the switch 46 connects the GPS altitude signal from the GPS22 to the LOOK-AHEAD warning generator 30. In this position, the GPSaltitude signal is generated either from the GPS 22 or from thedifferential GPS receiver 42, as discussed above. When a differentialGPS receiver 42 is not available, a compound altitude signal is providedby the block 44 to the LOOK-AHEAD warning generator 30. During thiscondition, the switch 46, under the control of a “DIFF GPS AVAILABLE”signal, connects the block 44 to the LOOK-AHEAD warning generator 30 toenable the compound altitude signal to be provided to the LOOK-AHEADwarning generator 30. Within the Differential GPC Receiver 42, thebarometric altitude is compared with the GPS altitude to generate acompound altitude signal which could be used to limit the maximumdifference between the GPS output altitude signal and the barometricaltitude to the maximum expected GPS altitude error. Such aconfiguration would reduce the effect of erroneous pressure correctionsettings on the barometric altimeter 28.

Terrain and Airport Data Bases

[0064] As mentioned above, the airport data and terrain data areseparated into two different data bases 26 and 24, respectively. Such aconfiguration allows either of the data bases 24 or 26 to be updatedwithout the need to update the other.

[0065] The airport data base 26 contains various types of informationrelating to airports, such as runway midpoint coordinates, runwaylength, runway heading, runway elevation and airport/runway designationdata. Additional data relating to obstacles along the approach path(i.e. high-rise hotels adjacent Heathrow Airport in England) and anominal final approach slope could also be included in the airport database 26. A separate obstacle data base may also be provided, structuredsimilar to the airport data base.

[0066] In order for the TAS 20 to be useful, all airports, for example,which commuter planes without radio altimeters can land, would have tobe included in the airport data base 26. In areas where the airport database 26 is not complete, the search-algorithm 29 would indicate NO DATA.Since the airport data base 26 is organized in a similar fashion as theterrain data base 24 as described below, additional airport data couldbe added in a piecewise fashion when available. The estimated size ofthe airport data base 26 is contemplated to be relatively small,compared to the terrain data base 24, for example less than 200kilobytes depending on the amount of data stored per airport.

[0067] The terrain data base 24 may require up to 40 megabytes ofstorage space, which makes it compatible with available flash erasableprogrammable read only memory (ROM) devices, such as a flash EEPROM, ormini hard disk drives, which could take advantage of known datacompression techniques to reduce the amount of storage space requiredand also promote easy data retrieval by the search algorithm 29.

[0068] An important aspect of the invention is that the terrain database 24 is structured to provide varying resolutions of terrain data asa function of the topography of the terrain, as well as distance toairports. For example, a relatively high resolution can be providedclose to the airport on the order of ¼ to ⅛ nautical miles and a mediumresolution, for example ½ to 1 nautical miles within a 30-mile radius ofthe airport. Outside of the 30-mile radius from the airport, a coarserresolution is sufficient as will be described below.

[0069]FIGS. 2 through 4 illustrate the organization of the terrain database. Referring first to FIG. 2, the world is divided into a pluralityof latitude bands 50, for example each about 4° wide. Each latitude band50 is then divided into a plurality of longitudinal segments 52, whichare about 4° wide around the equator, such that each longitudinalsegment 52 is about 256×256 nautical miles. In order to maintain arelatively constant segment size, the number of longitudinal segments 52per latitude band 50 are reduced closer to the poles.

[0070] Since the number of latitude bands 50 and the number oflongitudinal segments 52 per latitude band 50 is fixed, determination ofthe particular segment corresponding to the current aircraft position isreadily determined. For example, the aircraft current latitude X is usedto determine the latitude band number. Next, the number of longitudinalsegments 52 associated with that band 50 is determined either by way ofa LOOK-UP table or by calculation. Once the number of longitudinalsegments 52 in the particular latitude band 50 of interest isdetermined, the current longitudinal position Y of the aircraft can beused to readily determine the longitudinal segment number.

[0071] The minimum data associated with each longitudinal segment 52corresponds to the highest altitude within that segment. As mentionedabove, each of the segments 52 is approximately 256×256 nautical miles.As shown in FIG. 3, these longitudinal segments 52 can be broken downinto various subsquares to provide varying levels of resolution. Forexample, each segment 52 may be broken down into a plurality ofsubsquares 54, each subsquare 54 being 64×64 nautical miles to provide avery coarse resolution. The subsquares 54, can, in turn, be furthersubdivided into a number of subsquares 56, for example, each 16×16nautical miles, to provide a coarse resolution. These subsquares 56, inturn, can be subdivided into a plurality of subsquares 58, for example,each 4×4 nautical miles, to provide a medium resolution. The subsquares58 can also be broken down into a plurality of subsquares 60, forexample 1×1 nautical miles, to provide a fine resolution. In thevicinity around airports, it may even be desirable to break down thesubsquares 60 down into smaller subsquares 62 to provide even finerresolution, for example ¼×¼ nautical miles.

[0072] As shown in FIG. 4A, the minimum data associated with eachlongitudinal segment 52 consists of a header 65 which includes amultiple data byte 66 which includes the reference altitude whichcorresponds to the highest altitude for all the subsquares within thesegment, which assumes that the highest elevation in a square isrepresentative of the entire square elevation. The multiple data byte 66may also include a flag to indicate when no further subdividing isrequired for certain geographical areas. For example, for segmentsrepresenting the ocean, all subsquares would have the same maximumaltitude and thus no further subdividing would be required. In order toenable the data base to be created and updated on a piecewise basis, themultiple data byte 66 may also contain a flag bit or code indicatingthat data corresponding to further subdivisions does not exist for aparticular segment 52 containing a code indicating that no map dataexists in the segment.

[0073] For geographical areas, such as mountainous areas and areas inthe vicinity of an airport, the longitudinal segments 52 are subdividedas discussed above. In such a situation, the stored data would include a2-byte mask word 68, which points to the subsquares having differentaltitudes. For example, as shown in FIG. 4, the 2-byte mask word 68 isillustrated with a pointer in the box 15 within the subsquare 54 withinthe longitudinal segment 52, which represents a different altitude whichpoints to the next finer layer of resolution as discussed above. Thesubsquares each include a header 70 including a mask as described aboveto enable a pointer for each subsquare having a different altitude topoint to the next finer layer until the finest resolution level desireis reached as shown.

[0074] The data base structure provides for flexibility as to resolutionrequired and can be constructed on a piecewise basis at different times.As such, the structure contains the entire framework necessary to addnew data or modify existing data without changing any of the software.

[0075] The size of the data base is proportional to the resolution andthe size of the area to be covered. In order to cover the entire earth'ssurface, about 149 million square nautical miles, only about 2,500 to3,500 longitudinal segments 52 are required. Thus, the overhead for theminimum header sizes is relatively small.

[0076] Alternatively, the world may be divided into a plurality of, forexample, 1°×1° map files or cells 51 as shown in FIG. 4B. The variousmap files 51 may be formed with variable resolution. More particularly,each of the map files may be subdivided into a plurality of subcells 53with the highest terrain elevation in each subcell indicated, with theresolution of all of the subcells within a particular map file being ofthe same resolution. As indicated above, the maps are generally 1°×1°,which may be combined for coarser resolution (i.e. 2°×2° or 10°×10°)

[0077] The size of the subcells 53 is varied to provide varying degreesof resolution. For example, for relatively high resolution, the size ofthe subcells 53 may be selected as either 15×15 arc seconds or 30×30 arcseconds. The size of the subcells 53 may also be varied as a function ofthe terrain in particular geographic areas. For example, the size of thesubcells 53 may be selected as 30×60 arc seconds at relatively far northlongitudes and as 30×120 arc seconds at longitudes even further north.The size of the subcells 53 may also be varied as a function of thedistance to the nearest airport. For example, for distances greater than64 miles from an airport, the size of the subcells may be selected as60×60 arc seconds while at distances greater than, for example, 128miles from the airport, size of the subcell 53 may be selected as120×120 arc seconds. For a course resolution, for example for use duringa cruise mode as discussed below, the size of subcells may be selectedas 5×5 arc minutes.

[0078] As discussed below and as illustrated in FIG. 27, the data basemay be compressed by a compression algorithm and stored in a flash readonly memory (ROM), configured as a wedding cake as illustrated in FIG.28. Various compression algorithms are known and suitable for thisapplication. The compression algorithm does not form a part of thepresent invention.

[0079] In order to simplify computation, each of the map files includesa header 55, which includes, inter alia, the resolution of theparticular map file 51 as well as the location of one or more corners,for example, the northwest and southeast corners. The headers 55 as wellas the map files 51 are discussed in more detail below and illustratedin more detail in FIG. 29.

[0080] For certain terrain, for example, terrain adjacent a runway, theresolution of the-map files 51 (i.e. relatively large size of thesubcells 53) may result in errors in runway elevation. Moreparticularly, since each subcell 53 contains the highest elevationwithin the subcell 53, certain errors relating to runway elevations canoccur, depending on the resolution of the map file 51. Correction of theelevations adjacent the runways is discussed in more detail below inconjunction with FIGS. 30-33. Systematic errors may also be introducedby the maps, which are digitized, for example, by the Defense MappingAgency, using known averaging algorithms, which are known to fill invalleys and cut-off the tops of sharp peaks of the terrain.

LOOK-AHEAD Warning Generator

[0081] As mentioned above, the LOOK-AHEAD warning generator 30 generatesboth a terrain advisory signal and a terrain warning signal based uponthe position and trajectory of the aircraft relative to stored terraindata. There are two aspects of the terrain advisory and terrain warningsignals: LOOK-AHEAD distance/direction; and terrain threat boundaries.

LOOK-AHEAD Distance/Direction

[0082] The LOOK-AHEAD direction for detecting threatening terrain isalong the groundtrack of the aircraft. In order to prevent nuisancewarnings, the LOOK-AHEAD distance is limited, as will be discussedbelow. Otherwise, potentially threatening terrain along the currentflight path of the aircraft relatively far from its current positioncould produce nuisance warnings.

[0083] Two different LOOK-AHEAD distances (LAD) are utilized. The firstLAD is used for a terrain advisory signal (also referred to as a yellowalert LAD). A second LAD is used for terrain warning signals whichrequire immediate evasive action (also referred to as a red-alert LAD).

[0084] The LAD for a terrain advisory condition is considered first indetermining the LAD because it is assumed that the pilot could make a30° bank turn at any time at a turning radius R. The LAD is equal to theproduct of the speed of aircraft and the total LOOK-AHEAD time. As shownin FIG. 5, the total LOOK-AHEAD time is equal to the sum of theLOOK-AHEAD time T₁ for a single turning radius R; the LOOK-AHEAD time T₂for terrain clearance at the top of the turn (i.e. point 73) plus apredetermined reaction T₃.

[0085] The terrain clearance at the top 73 of the turn is provided toprevert inadvertent terrain contact as a result of the turn. Thisterrain clearance may be selected as a fixed distance or may be madeequal to the turning radius R.

[0086] As shown in equation (1), the turning radius R is proportional tothe square of the speed of the aircraft and inversely proportional tothe bank angle TG (ROLL).

R=V ² /G×TG(Roll)  (1)

[0087] For a 30° bank angle, the turning radius R in nautical miles (nM)as a function of the speed in knots is given by equation (2).

R=0.000025284*V ²  (2)

[0088] The LOOK-AHEAD time T₁ for a single turning radius is given byequation (3).

T ₁ =R/V  (3)

[0089] Substituting R from equation (1) into equation (3) yields theLOOK-AHEAD time T₁ for a single turn radius as shown in equation (4) asa function of the speed of the aircraft and the bank angle.

T=V/G×TG(Roll)  (4)

[0090] TABLE 1 provides various LOOK-AHEAD times T₁ at various turningradii and ground speeds. TABLE 1 Radius Speed T₁ (nM) (knots) (sec) ½140 13 1 200 18 2 280 26 3 345 31 4 400 38 5 445 40 6 490 44

[0091] By assuming that the fixed terrain clearance at the top 73 of theturn is equal to one turning radius R, the total LOOK-AHEAD time for twoturn radii (i.e. T₁+T₂) is simply twice the time for a single turningradius. Thus, the total LOOK-AHEAD time is 2*T₁ plus the predeterminedreaction time T₃, for example 10 seconds, as given by equation (5).

T _(TOTAL)=2*T ₁ +T ₃  (5)

[0092] By substituting R from equation (2) into equation (4), theLOOK-AHEAD time for a single turn radius is given by equation (6).

T ₁=0.000025284*V  (6)

[0093] Thus, if it is assumed that the fixed clearance X at the top ofthe turn 73 is also equal to the turning radius R, the total LOOK-AHEADtime for two turning radii is given by equation (7) below, simply twicethe value determined in equation (6)

T ₁ +T ₂=2*0.000025284*V  (7)

[0094] For a predetermined reaction time T₃, for example 10 seconds, theLOOK-AHEAD distance (LAD) in nautical miles for a terrain advisorysignal can be determined simply by multiplying the speed of the aircraft(V) by the total time T_(TOTAL) as shown in equation (8).

LAD=V*(0.0000278+2*0.000252854*V)+fixed distance*K,  (8)

[0095] where K is a constant, for example 0.

[0096] However, in order to provide optimal conditions during a terrainadvisory condition, the LAD is limited with an upper limit and a lowerlimit. The lower limit may be a configurable amount, for example, either0.75, 1 or 1-½ nautical miles at relatively low speeds (i.e. speeds lessthan 150 knots) and limited to, for example, 4 nautical miles at speeds,for example, greater than 250 knots. The LAD may also be limited to afixed amount regardless of the speed when the distance to the runway isless than a predetermined amount, for example 2 nautical miles, exceptwhen the aircraft altitude is greater than 3,000 feet, relative to therunway.

[0097] The terrain warning LAD is given by equation (9) below.

LAD=k ₁ *LAD(terrain for LOOK-DOWN/LOOK-AHEAD advisory indication),

k₂ *LAD(terrain LOOK-DOWN/LOOK-AHEAD warning),

k ₃ *LAD(terrain LOOK-UP advisory),  (9)

[0098] where k₁=1.5, except when the LAD is limited at its lower limit,in which case k₁=1, k₃=1 and where k₂=0.5, k₃=2.

Terrain Threat Boundaries

[0099] The terrain threat boundaries include a terrain floor boundary,terrain advisory boundaries (yellow alert), as well as terrain warningboundaries (red alert) and are provided along the groundtrack of theaircraft.

[0100] Terrain Floor Boundary

[0101] The terrain floor boundary is the basis for the terrain threatboundaries and is similar to the terrain floor developed for the GPWS.The terrain floor relates to a distance ΔH below the aircraft and isproportional to the distance to the closest runway to prevent nuisancewarnings when the aircraft is taking off and landing, while providingadequate protection in other modes of operation. As illustrated inequation (10), the terrain floor boundary below the aircraft isessentially based upon providing 100 feet clearance per nautical milefrom the runway, limited to 800 feet.

ΔH=100(ft/nM)*(distance to runway threshold−anoffset)+100(ft/nM)*(distance to runway center−12 nautical miles)  (10)

[0102] Equation (10) is shown graphically in FIG. 6. Referring to FIG.6, the horizontal axis represents the distance from the runway, whilethe vertical axis represents the ΔH terrain floor boundary beneath theaircraft. The first segment 71 represents the runway length, while thepoint 72 represents the runway center. After a small offset 74, the nextsegment 78, which starts at 0 feet, slopes at 100 feet per nautical miledistance from the runway threshold, identified by the point 80, up to amaximum of 500 feet. The 500 foot maximum continues along a segment 82until the distance to the runway center 72 is a predetermined distanceD, for example 12 nautical miles. The next segment 84 slopes up at a 100feet per nautical mile distance until the distance rises 300 feet fromthe segment 82 for a total of 800 feet.

[0103] The terrain floor ΔH boundary beneath the aircraft is limitedsuch that the segment 78 begins at 0 and the segment 82 never goes abovea predetermined maximum, for example 500 feet. In addition, the terrainfloor ΔH boundary is also limited so that the segment 84 never goesbelow the vertical height of the segment 82 and rises no more than 300feet relative to the segment 82. During conditions when the aircraft is3,000 feet above the terrain, the terrain floor ΔH boundary ismaintained at the upper limit of 800 feet.

[0104] Terrain Advisory Boundaries

[0105] Two terrain advisory boundaries are shown graphically in FIGS. 7and 8. FIG. 7 represents a condition when the aircraft is descendingwhile FIG. 8 represents a condition when the aircraft is ascending. Aswill be discussed in more detail below, the terrain advisory boundariesare based upon the relationship between the flight path angle γ and afirst configurable datum, THETA1.

[0106] Referring first to FIG. 7, the terrain advisory (yellow alert)boundaries are shown. The first segment of the terrain advisoryboundary, identified with the reference numeral 92, corresponds to theΔH terrain floor boundary. As mentioned above, the ΔH terrain floorboundary is a function of the distance from the runway 94. In order todetermine the bottom segment 100 of the terrain advisory boundary, it isnecessary to determine whether the flight path angle γ is more or lessthan a configurable datum, identified as THETA1. As shown and describedherein, THETA1 is set for 0°. However, it should be clear that otherangles for THETA1 are within the broad scope of the present invention.For the condition illustrated in FIG. 7, the flight path angle γ is lessthan THETA1. Thus, the segment 100 extends from the ΔH terrain floorboundary at the angle THETA1 to the LOOK-AHEAD distance for a terrainadvisory. The final segment 102 is generally parallel to the segment 92and extends in a vertical direction along the LAD for the terrainadvisory boundary.

[0107] If the flight path angle γ is greater than THETA1 as shown inFIG. 8., then different terrain advisory boundaries are provided. Thesegment 92 will be the same as shown in FIG. 7 and described above.However, when the flight path angle γ is greater than THETA1 as shown inFIG. 8, a bottom segment 106 is formed by extending a line segment fromthe bottom of the segment 92, which represents the ΔH terrain floor,along a direction parallel with the flight path angle γ up to the LAD. Avertical segment 108 extends from the bottom segment 106 along the LADto form the terrain advisory boundaries.

[0108] Thus, as shown, if the airplane is climbing, only terrain whichpenetrates the upward-sloping terrain advisory boundaries will cause anadvisory signal, such that if the aircraft clears the terrain by a safemargin, no advisory signal is given, even if the terrain ahead is at orabove the present aircraft altitude. However, if terrain penetrates theterrain advisory boundaries, an aural warning, such as “CAUTION,TERRAIN” aural advisory may be given. In addition, visual warnings maybe provided.

[0109] Terrain Warning Boundaries

[0110] The terrain warning boundaries are shown in FIGS. 9 and 10 and,in general, indicate to the pilot of an aircraft conditions when evasiveaction is required to avoid terrain contact. The terrain warningboundaries are based on the relationship of the flight path angle of theaircraft relative to a second configurable datum, THETA2. The datumTHETA2 may be selected with an upslope of, for example, 6°, which isequal to the average climb capability of airliners. The datum THETA2could be modified, taking into consideration aircraft type,configuration, altitude and time for takeoff. However, because of thelonger LOOK-AHEAD distance as discussed above for the terrain warningboundaries, a terrain warning could occur before a terrain advisoryindication for extreme terrain conditions.

[0111] Referring to FIGS. 9 and 10, two different terrain warningboundaries are illustrated. In particular, FIG. 9 illustrates acondition when the flight path angle γ is greater than the second datumTHETA2. FIG. 10 illustrates a condition when the flight path angle γ isless than the second datum THETA2. In both conditions, the first segment114 of the terrain warning boundary is extended below the aircraft for adistance ½ of the ΔH terrain floor, which, as mentioned above, is afunction of the distance of the aircraft from a runway. As mentionedabove, the terrain warning boundaries are dependent on the relationshipbetween the flight path angle γ of the aircraft and the datum THETA2.

[0112] Referring first to FIG. 9, since the flight path γ is less thanthe second datum THETA2, the bottom segment 118 of the terrain warningboundary is formed by extending a line from the segment 114 at an angleequal to the flight path angle γ up to the LOOK-AHEAD distance for aterrain warning as discussed above. A vertical segment 120 extends fromthe lower segment 118 along the LAD for a terrain warning, forming theterrain warning boundaries illustrated in FIG. 9.

[0113] If the flight path angle of the aircraft is less than the slopeof the second datum THETA2 as shown in FIG. 10, a bottom segment 122extends from the segment 114 up to the LOOK-AHEAD distance at an angleequal to the flight path angle γ. A segment 124 extends upwardly alongthe LAD from the segment 122. If any terrain along the groundtrack ofthe aircraft out to the LAD for the terrain warning penetrates theenvelope, aural and/or visual terrain ahead warnings may be given.

[0114] Cut-Off Boundaries

[0115] In order to avoid spurious warnings when the aircraft overflies aridge at relatively low altitudes, the warning boundaries may includecut-off boundaries, for example, as illustrated in FIGS. 11, 12 and 13.Without the cut-off boundaries, warnings would be given, although theterrain is virtually below the aircraft and no terrain is visible ahead.Referring first to FIG. 11, the cut-off boundary 126 begins at apredetermined cut-off offset 128 below the aircraft and extends in adirection in front of the aircraft at a predetermined envelope cut-offangle 130. The envelope cut-off angle 130 is equal to the flight pathangle γ plus a configurable predetermined cut-off angle, described andillustrated as −6°. For level flight as shown in FIG. 11, the cut-offboundary 126 extends from the cut-off offset 128 in the direction of theenvelope cut-off angle 130 toward the front of the aircraft to a point132 where it intersects a terrain advisory boundary or terrain warningboundary, identified with the reference numeral 134. For level flight,as shown in FIG. 11, the flight path angle γ is zero. Thus, the cut-offboundary 126 illustrated in FIG. 11 will extend from the cut-off offset128 along an angle equal to the cut-off angle, which, as mentionedabove, is selected as −6° for illustration. As mentioned above, thecut-off boundary 126 extends from the cut-off offset 128 to the point132 where it intersects the terrain advisory boundary 134 discussedabove. The warning boundary is then selected to be the highest of theterrain advisory boundary 134 and the envelope cut-off boundary 126.Thus, for the example illustrated in FIG. 11, the terrain advisoryboundary would consist of the cut-off boundary 126 up to the point 132,where the envelope cut-off boundary 124 intersects the warning envelope126. From the point 132 forward, the normal terrain advisory boundary134, corresponding, for example, to a THETA1 slope, is utilized. Thus,if either a terrain advisory boundary or terrain warning boundary isbelow the cut-off boundary 126, the cut-off boundary 126 becomes the newboundary for the advisory or warning signal.

[0116] The cut-off angle 130 is limited such that if the flight pathangle γ is a relatively high climb angle, the cut-off angle 130 does notexceed a predetermined limit, for example a configurable limit between 0to 6° as will be illustrated in FIG. 12. In particular, referring toFIG. 12, the flight path angle γ is shown at a relatively high climbrate (i.e. γ=9°). An unlimited cut-off envelope angle 130 is illustratedby the partial segment 136. As shown, this segment 132 is 6° below theflight path angle γ However, in order to decrease the sensitivity of thesystem 20 during conditions when the flight path angle is relativelylarge, for example 9°, indicative of a fairly steep climb, the cut-offenvelope angle 130 is limited. In the example illustrated in FIG. 12,the cut-off angle limit is selected, for example, to be 1.4°. Thus, atflight path angles γ greater than 7.4°, the cut-off angle will result inan envelope cut-off boundary 140, which is more than 6° below the flightpath angle γ. As shown, since the cut-off angle limit was selected as1.4°, the cut-off angle will be limited to 1.4° anytime the flight pathangle is greater than 7.4°, even though the cut-off angle will be equalto or more than 6° less than the flight path angle γ. Thus, for theconditions illustrated in FIG. 12, the segment 140 begins at the cut-offoffset 128 and continues along a line that is 1.4° from the horizonuntil it intersects the terrain advisory or terrain warning boundary 142at the point 144. The actual advisory or warning boundary will be formedby the limited cut-off boundary 140 up to the point 144 and willcontinue with the normal warning boundary 142 beyond the point 144.

[0117]FIG. 13 illustrates a situation when the aircraft is descendingand thus the flight path angle γ is less than 0. During conditions whenthe flight path angle γ is less than the normal descent angle, thewarning, boundary is lowered past the cut-off boundary for highsink-rate situations in order to prevent nuisance warnings duringstep-down approaches. In particular, a BETA sink rate enhancement isprovided and is effective anytime the flight path γ is below apredetermined configurable descent bias GBBIAS, for example, −4°. Duringsuch conditions, an angle BETA is added to the terrain advisory datum(THETA1) as indicated below. In particular, when the flight path angle γis greater than the descent bias angle GBBIAS or the flight path angle γis less than zero, BETA is selected as 0. When the flight path angle γis greater than zero, BETA is given by equation (11).

BETA=γ−THETA1; for y>0  (11)

[0118] When the flight path angle γ is less than GBBIAS, the value forBETA is selected in accordance with equation (12) below.

BETA=K*(γ−GBBIAS),  (12)

[0119] where k=0.5, γ=flight path angle and GBBIAS is selected as −4°.

[0120]FIG. 13 illustrates a condition when the flight path angle γ is−8°. During this condition, the cut-off angle 130 is selected to be 6°below the flight path angle γ and thus defines an envelope cut-offboundary 150 that extends from the cut-off offset 128 along a line whichis −14° from the horizontal axis. Since the flight path angle γ is lessthan GBBIAS, the angle BETA is added to the normal warning boundaryaccording to equation (12) above. Thus, for a flight path angle of −8°,the BETA angle will be 2°. Thus, the terrain advisory or warningboundary, indicated by the boundary 151, instead of extending along theTHETA1 slope, will extend along an angle which is 2° below the THETA1slope to define the enhanced boundary 152. The warning cut-off boundary152 thus will extend to the point 156 where it intersects the normalterrain advisory or terrain warning boundary 152. Since the cut-offangle boundary 150 is higher than boundary 152 up to the point 156, thecut-off boundary 150 will be utilized as the warning boundary up to thepoint 156. Beyond point 156, the BETA-enhanced warning boundary 152 willdefine the warning envelope.

[0121]FIGS. 14, 15 and 16 illustrate the composite terrain advisory andterrain warning boundaries for different conditions. The cross-hatchedarea illustrates terrain in the vicinity of the aircraft. Threeconditions are illustrated. In particular, FIG. 14 represents acondition when the flight path angle γ of the aircraft is less than thefirst datum THETA1. FIG. 15 represents a condition when the flight pathangle γ is between the first datum THETA1 and the second datum THETA2.FIG. 16 represents a condition when the flight path angle γ is greaterthan the second datum THETA2. The terrain advisory boundaries areindicated in solid line while the terrain warning boundaries areillustrated by dashed lines.

[0122] Referring first to FIG. 14, the terrain advisory boundariesinclude a boundary 155, which extends from the aircraft to the ΔHterrain floor. Normally, the lower terrain advisory boundary 157 wouldextend from the segment 155 along an angle equal to the THETA1 slope.However, in this situation, the flight path angle γ is illustrated to begreater than the GBBIAS angle; therefore, a BETA-enhanced boundary 159,as discussed above, is utilized which extends along an angle that isequal to the THETA1 angle plus the BETA angle, up to the LOOK-AHEADdistance for a terrain advisory. A vertical boundary 158 extends fromthe first datum THETA1 vertically upwardly along the LOOK-AHEAD distancefor a terrain advisory. Since the cut-off boundary 157 is higher thanthe terrain advisory boundary 157 up to the point 169 where the twoboundaries intersect, the cut-off boundary 157 becomes the terrainadvisory boundary up to the point 169. Beyond the point 169, theboundary 159 is utilized as the warning boundary.

[0123] The terrain warning boundary includes a boundary 162 extending toa point equal to ½ of the terrain floor as discussed above. A boundary165 extends from the boundary 162 at the THETA2 slope. A verticalboundary 167 extends from the boundary 165 along the LOOK-AHEAD distancefor a terrain warning as discussed above.

[0124]FIG. 15 illustrates a situation when the flight path angle γ isgreater than the THETA1 angle and less than or equal to the THETA2angle. During this condition, the boundary for the terrain warningextends along a segment 180, which is at ½ of the ΔH terrain floor. Asloping boundary 184 extends from the segment 180 to the LOOK-AHEADdistance for a terrain warning (point 186). From there, a verticalboundary 188 extends vertically upwardly to define the terrain warningboundaries.

[0125] The terrain advisory boundary includes a boundary 185 whichextends downwardly to the terrain floor, i.e. point 187. From point 187,the terrain advisory boundary extends along a segment 189 at the flightpath angle γ up to a point 191, the LOOK-AHEAD distance for a terrainadvisory. A vertical segment 194 extends along the LAD from the boundary189 at the point 191.

[0126] In this illustration, an envelope cut-off boundary 193 cuts off aportion of the terrain advisory indication boundary since the flightpath angle γ of the aircraft is greater than a predetermined limit.Thus, the portions of the warning boundary 189 below the cut-off angleboundary 193 are ignored; and the segment 193 becomes the effectiveterrain advisory indication up to the point 195, where the cut-offboundary 193 intersects the terrain advisory indication boundary 189.After the point 155, the boundary 189 will be the effective warningboundary.

[0127]FIG. 16 illustrates a situation when the flight path angle γ isgreater than THETA2. In this situation, a terrain advisory boundary 200extends from the aircraft to the boundary terrain floor. A lowerboundary 202 extends at the flight path angle γ from the boundary 200 upto the LOOK-AHEAD distance for a terrain advisory. A vertical terrainadvisory boundary 204 extends upwardly along the LOOK-AHEAD distance forthe terrain advisory. Since the flight path angle γ is illustrated tothe greater than GBBIAS, a segment 211, the cut-off boundary, definesthe effective warning boundary up to a point 213 where the terrainadvisory boundary intersects the cut-off warning boundary. After thatpoint, segment 202, which is higher, becomes the warning boundary.

[0128] The terrain warning boundary extends along a segment 208 down toa point of ½ the terrain floor. A lower boundary 210 extends along anangle equal to flight path angle γ since it is greater than the THETA2slope. A vertical terrain warning boundary 212 extends upwardly from theboundary 210 along the LOOK-AHEAD distance for a terrain warning.

[0129] In order to provide additional sensitivity of the warning system,terrain advisory signals within a predetermined portion of the LAD for aterrain advisory, for example ½ LAD, may be treated as terrain warnings.Thus, as shown in FIGS. 14, 15 and 16, vertical boundaries 161, 181 and201, respectively, are shown, for example, at ½ LAD. Based upon theflight path angles shown, these segments define effective terrainadvisory envelopes 166, 182 and 203.

Alternative Terrain Threat Boundaries

[0130] Alternative terrain threat boundaries are illustrated in FIGS.17-22. Similar to the terrain threat boundary discussed above andillustrated in FIGS. 5-16, the alternative terrain threat boundariesinclude a terrain floor boundary, terrain advisory boundaries (yellowalert) and terrain warning boundaries (red alert). In the alternativeembodiment, the advisory and warning boundaries are further divided intotwo parts, a LOOK-AHEAD/LOOK-DOWN boundary for detecting terrain aheador below the aircraft (FIGS. 17 and 18) and a LOOK-UP boundary fordetecting precipitous high terrain ahead of the aircraft which may bedifficult to clear (FIG. 19).

LOOK-AHEAD/LOOK-DOWN Terrain Advisory and Warning Boundaries

[0131] As mentioned above, the LOOK-AHEAD/LOOK-DOWN boundaries areillustrated in FIGS. 17 and 18. Referring first to FIG. 17,LOOK-AHEAD/LOOK-DOWN advisory and warning boundaries are illustrated fora condition when the aircraft is descending (i.e. γ<0) During such aconfiguration, the first segment of the LOOK-AHEAD/LOOK-DOWN terrainadvisory boundary, identified with the reference numeral 300,corresponds to the ΔH terrain floor boundary. As discussed above, the ΔHterrain floor boundary is a function of the aircraft from the runway. Inorder to determine the bottom segment 302 of the LOOK-AHEAD/LOOK-DOWNterrain advisory boundary, the flight path angle γ is compared with aconfigurable datum, THETA1, for example 0°. During descent conditions,the flight path angle γ will thus be less than THETA1. Thus, theLOOK-AHEAD/LOOK-DOWN terrain advisory boundary segment will extend fromthe ΔH terrain floor boundary segment 300 along the angle THETA1 to thelook-ahead distance for a terrain advisory (LAD). The final segment 304extends vertically upwardly from the segment 302 along the LAD.

[0132] The LOOK-AHEAD/LOOK-DOWN terrain advisory boundary may also bemodified by the BETA sink rate enhancement as discussed above. In thisembodiment, the BETA sink rate enhancement ensures that an advisoryindication always precedes a warning indication when the aircraftdescends into or on top of terrain. The BETA sink rate enhancement isdetermined as a function of the flight path angle γ and two (2)configurable constants KBETA and GBIAS. The BETA sink rate enhancementBETA1 for a LOOK-AHEAD/LOOK-DOWN terrain advisory is provided inequation (13) below for a condition when the aircraft is descending.

BETA 1=KBETA*(γ−GBIAS),  (13)

[0133] where GBIAS is a configurable constant, selected for example, tobe zero (0) and KBETA is also a configurable constant selected, forexample, to be 0.5.

[0134] Referring to FIG. 17, the BETA sink rate enhancement BETA1 forthe LOOK-AHEAD/LOOK-DOWN terrain advisory boundary provides an advisorywarning at a distance less than ½ LAD, unlike the terrain advisoryboundary described above). More particularly, for the values definedabove for the configurable constants KBETA and GBIAS, the BETA sink rateenhancement BETA1 results in a segment 306 which extends from the ΔHterrain floor segments 300 at an angle equal to γ/2 up to ½ of the LAD.Beyond ½ LAD, a segment 308 extends at the angle THETA1 to a distanceequal to the LAD. A vertical segment 310 extends along the LAD toconnect the segments 308 to the segment 304.

[0135] As discussed above, a cut-off boundary may be provided to reducenuisance warnings. Referring to FIG. 17, the cut-off boundary isidentified with-the reference numeral 312. The cut-off boundary 312extends from a vertical distance 314 below the aircraft along a cut-offangle up to the point of intersection 316 with the terrain advisoryboundary. For distances less than the intersection 316, the cut-offboundary 312 forms the terrain advisory boundary. For distances beyondthe point of intersection 316, the boundaries 306 and 308 form theterrain advisory boundaries.

[0136] The terrain warning boundary includes the segment 300 extendingfrom the aircraft along the ΔH terrain floor. A bottom segment 318connects to the segment 300 and extends along a BETA sink rateenhancement angle BETA2. The BETA sink rate enhancement angle isdetermined as a function of the flight path angle γ and a configurableconstant KBETA2 as well as the constant GBIAS discussed above andprovided in equation (14) below.

BETA 2=KBETA 2*(GAMMA−GBIAS),  (14)

[0137] where GBIAS is a configurable constant selected, for example, tobe 0 and KBETA2 is also a configurable constant selected, for example,to be 0.25.

[0138] For such values of the constants KBETA2 and GBIAS, the BETAenhancement angle KBETA2 will be ¼*γ. Thus, the segment 318 extends fromthe segment 300 at an angle equal to ¼*γ up to ½ the LAD. A verticalsegment 320 extends along a distance equal to ½*LAD from the segment 318to define the terrain warning boundary.

[0139] Similar to the above, the terrain warning boundaries are alsolimited by the cut-off boundary 312. Thus, the cut-off boundary 312forms the terrain warning boundary up to a point 322, where the cut-offboundary 312 intersects the lower terrain warning boundary 318. Atdistances beyond the point of intersection 322, the segment 318 formsthe lower terrain warning boundary up to a distance equal to ½ of theLAD.

[0140] The terrain advisory and terrain warning boundaries for acondition when the aircraft is climbing (i.e. γ>0) is illustrated inFIG. 18. During such a condition, the BETA sink rate enhancement anglesBETA1 and BETA2 are set to a configurable constant, for example, zero(0).

[0141] The terrain advisory boundary during a climbing condition isformed by extending a vertical segment 324 from the aircraft for adistance below the aircraft equal to the ΔH terrain floor. During aclimbing condition, a segment 326 is extended from the segment 324 tothe LAD at an angle equal to the flight path angle γ. At a point 328where the segment 326 intersects a position equal to ½ of the LAD, avertical segment 330 is extended up from the segment 326, forming afirst vertical boundary for the terrain advisory condition. The linesegment 326 from the point 328 to the LAD forms the lower terrainadvisory boundary while a line segment 332 extending vertically upwardfrom the line segment 326 along the LAD forms a second verticalboundary.

[0142] For the exemplary condition illustrated, a cut-off boundary 334does not intersect the terrain advisory boundaries. Thus, the terrainadvisory boundaries for the exemplary condition illustrated is formed bythe segments 330 and 332 and that portion of the line segment 326between the line segments 330 and 332.

[0143] The terrain warning boundaries for a condition when the aircraftis climbing includes the vertical segment 324 (FIG. 18) which extendsfrom the aircraft to vertical distance equal to the ΔH terrain floorbelow the aircraft forming a first vertical boundary. For a conditionwhen the aircraft is climbing, the line segment 326 extends from thesegment 324 at the flight path angle γ to form the lower terrain warningboundary. The vertical segment 330 at a distance equal to ½ of the LADforms the second vertical terrain warning boundary.

[0144] The cut-off boundary 334 limits a portion of the terrain warningboundary. In particular, the cut-off boundary 334 forms the lowerterrain warning boundary up to a point 340, where the cut-off boundary334 intersects the line segment 326. Beyond the point 340, a portion 342of the line segment 340 forms the balance of the lower terrain warningboundary up to a distance equal to ½ of the LAD.

LOOK-UP Terrain Advisory and Warning Boundaries

[0145] The LOOK-UP terrain advisory and terrain warning boundaries areillustrated in FIG. 19. As will be discussed in more detail below, theLOOK-UP terrain advisory and warning boundaries start at altitudesDHYEL2 and DHRED2, respectively, below the aircraft. These altitudesDHYEL2 and DHRED2 are modulated by the instantaneous sink rate (i.e.vertical speed, HDOT of the aircraft). The amount of modulation is equalto the estimated altitude loss for a pull-up maneuver, for example, at¼G (i.e. 8 ft/sec²) at the present sink rate. The altitudes DHRED2 andDHYEL2 are dependent upon the altitude loss during a pull-up maneuver(AlPU) and the altitude loss due to reaction time (ALRT) discussedbelow.

[0146] The estimated altitude loss due to a pull-up maneuver ALPU isbest understood with reference to FIG. 20, wherein the vertical axisrelates to altitude in feet and the horizontal axis relates to time inseconds. The trajectory of the aircraft from a time shortly beforepull-up is initiated to a time when the aircraft has recovered is shownand identified with the reference numeral 344.

[0147] Assuming an advisory or warning indication is generated at timeT₁ at a point 346 along the trajectory 344, the altitude loss due to thereaction time of the pilot (ALRT) is given by equation (15) below.

ALRT=HDOT*T _(R),  (15)

[0148] where HDOT equals the vertical acceleration of the aircraft infeet/sec and T_(R) equals the total reaction time of the pilot inseconds.

[0149] Assuming a pull-up maneuver is initiated at time T₂, the altitudeloss due to the pull-up maneuver ALPU may be determined by integratingthe vertical velocity HDOT with respect to time as set forth in equation(16) below.

HDOT(t)=a*t+HDOT ₀,  (16)

[0150] where “a” equals the pull-up acceleration and HDOT₀ is aconstant.

[0151] Integrating both sides of equation (16) yields the altitude lossas a function of time H(t) as provided in equation (17) below.

H(t)=½*a*t ² +HDOT ₀ t  (17)

[0152] Assuming a constant acceleration during the pull-up maneuver, thetime t until vertical speed reaches zero (point 348) is given byequation (18).

t=−HDOT ₀ /a  (18)

[0153] Substituting equation (18) into equation (17) yields equation(19).

ALPU=−(HDOT ₀)²/(2*a)  (19)

[0154] Equation (19) thus represents the altitude loss during thepull-up maneuver.

[0155] An exemplary block diagram for generating the signals ALTR andALPU is illustrated in FIG. 21. In particular, a signal representativeof the vertical velocity of the aircraft HDOT, available, for example,from a barometric altimeter rate circuit (not shown), is applied to afilter 350 in order to reduce nuisance warnings due to turbulence. Thefilter 350 may be selected with a transfer function of 1/(TAHDOT*S+1);where TAHDOT is equal to one second. The output of the filter 350 is asignal HDOTf, which represents the filtered instantaneous verticalspeed; positive during climbing and negative during descent.

[0156] In order to generate the altitude loss due to reaction timesignal ALTR, the signal HDOTf is applied to a multiplier 352. Assuming apilot reaction time Tr, for example, of 5 seconds, a constant 354 equalto 5 seconds is applied to another input of the multiplier 352. Theoutput of the multiplier 352 represents the signal ALTR, which ispositive when HDOTf is negative and set to zero if the signal HDOTf ispositive, which represents a climbing condition. More particularly, thesignal HDOTf is applied to a comparator 356 and compared with areference value, for example, zero. If the comparator 356 indicates thatthe signal HDOTf is negative, the signal HDOTf is applied to themultiplier 352. During climbing conditions, the signal HDOTf will bepositive. During such conditions, the comparator 356 will apply a zeroto the multiplier 352.

[0157] The altitude loss due to the pull-up maneuver signal ALPU isdeveloped by a square device 358, a divider 360 and a multiplier 362.The filtered instantaneous vertical speed signal HDOTf is applied to thesquare device 358. Assuming a constant acceleration during the pull-upmaneuver, for example, equal to 8 feet/sec² (0.25 G), a constant isapplied to the multiplier 362 to generate the signal 2 a. This signal, 2a, is applied to the divider 360 along with the output of the squaredevice 350. The output of the divider 360 is a signal (HDOT f)²/2 a,which represents the altitude loss during a pull-up maneuver signalALPU.

[0158] These signals ALRT and ALPU are used to modulate the distancebelow the aircraft where terrain advisory and terrain warning boundariesbegin during a LOOK-UP mode of operation. More particularly, during sucha mode of operation, ΔH terrain floor segment of the terrain advisoryboundary DHYEL2 is modulated by the signals ALRT and ALPU while the ΔHterrain floor segment of the terrain warning boundary DHRED2 ismodulated by the signal ALPU as indicated in equations (20) and (21),respectively.

DHYEL 2={fraction (3/4)}*ΔH+ALRT+ALPU  (20)

DHRED 2={fraction (1/2)}*Δ+ALPU,  (21)

[0159] where ΔH represents the terrain floor as discussed above.

[0160] Thus, referring to FIG. 19, the LOOK-UP terrain advisory warningbegins at a point 364 below the aircraft; equal to DHYEL2. If the flightpath angle γ is less than a configurable datum THETA2, a terrainadvisory boundary 366 extends from the point 364 to the advisory LAD atan angle equal to THETA2. Should the flight path angle γ be greater thanTHETA2, the lower advisory boundary, identified with the referencenumeral 368, will extend from the point 364 at an angle equal to theflight path angle γ.

[0161] Similarly, the LOOK-UP terrain warning boundary begins at point370 below the aircraft; equal to DHRED2. If the flight path angle γ isless than the angle THETA2, a warning boundary 372 extends from thepoint 370 at angle THETA2 to the warning LAD. Should the flight pathangle γ be greater than THETA2, a warning boundary 374 will extend at anangle equal to the flight path angle γ between the point 370 and thewarning LAD.

Cut-Off Altitude

[0162] The cut-off altitude is an altitude relative to the nearestrunway elevation; set at, for example, 500 feet. Altitudes below thecut-of altitude are not displayed and are ignored by the terrainadvisory and terrain warning computations.

[0163] The use of a cut-off altitude declutters the display discussedbelow around airports, especially during a final approach when theaircraft approaches the ground.

[0164] A second advantage is that nuisance warnings on a final approach,due to altitude errors, terrain data base resolution and accuracy errorsare minimized.

[0165] However, the use of a cut-off altitude during certain conditions,such as an approach to an airport on a bluff (i.e. Paine Field) at arelatively low altitude or even at an altitude below the airportaltitude, may compromise system performance. More particularly, duringsuch conditions, the use of a cut-off altitude may prevent a terrainwarning from being generated because the threatening terrain may bebelow the cut-off altitude. In order to account of such situations, thesystem selects the lower of two cut-off altitudes; a nearest runwaycut-off altitude (NRCA) and a cut-off altitude relative to aircraft(CARA). The NRCA is a fixed cut-off altitude, relative to the nearestrunway. The CARA is an altitude below the instantaneous aircraftaltitude (ACA) by an amount essentially equivalent to ΔH terrain floorboundary for an advisory condition, which, as discussed above, is afunction of the aircraft distance to the nearest runway.

[0166] Equations (22) and (23) below set forth the NRCA and CARA. Asmentioned above, the absolute cut-off altitude (ACOA) is the lower ofthe NRCA and CARA as set forth in equation (24).

NRCA=COH+RE,  (22)

[0167] where COH relates to the cut-off height and is a fixedconfigurable value, initially set between 400 feet and 500 feet; and REequals the runway elevation.

CARA=ACA−ΔH−DHO,  (23)

[0168] where ACA is the instantaneous aircraft altitude; ΔH is theterrain floor which is proportional to the distance to the runway asdiscussed above; and DHO is a configurable bias, set to, for example, 50feet.

ACOA=lower of CARA, NRCA,  (24)

[0169] except where ΔH DH1, in which case ACOA is always equal to NRCA.

[0170] DH1 is a point at which the ACOA is forced to be equal to NRCAindependent of the aircraft altitude. The point DH1 is related to COH,ΔH and DHO such that on a nominal three (3) degree approach slope, CARAis equal to NRCA when the aircraft is at a distance equal to a distanceDH1 from the airport, as illustrated in TABLE 2 below. TABLE 2 COH DH1DISTANCE TO (feet) (feet) RUNWAY (n mile) 300 50 1 400 100 1.5 500 150 2

[0171] The point DH1 forces the cut-off altitude (COH) above the runwaywhenever the aircraft is close to the runway to ensure robustnessagainst nuisance warnings caused by altitude errors and terrain database resolution margins to disable the terrain advisory and warningindications when the aircraft is within the aircraft perimeter. Thereare trade-offs between nuisance warnings and legitimate warnings. Inparticular, the lower COH, the closer the advisory and warningindications are given, making the system more nuisance prone. Asindicated above, for a COH of 400, terrain advisory and warningindications are effectively disabled when the aircraft is closer than1.5 n miles from the airport runway.

[0172]FIG. 22 illustrates the operation of the alternative cut-offaltitude boundaries. In particular, FIG. 22 illustrates a condition whenthe COH is set to 300 feet with DH1 equal to 50 feet. The cut-offaltitude substantially for area from the runway, for example, greaterthan 4 nautical miles, is 300 feet, as indicated by the segment 378 whenthe glide slope angle is less than a predetermined angle, for example,3°. As the aircraft gets closer to the runway, the COH is lowered, asillustrated by the segment 388, until the aircraft is within one (1)nautical mile of the runway, at which point the COH is forced to be 300feet, which effectively disables any terrain advisory and warningindications within the aircraft is closer than one (1) nautical milefrom the runway, as represented by the segment 382.

[0173] During a condition when the aircraft is on a, for example, 3°glide slope angle, the ACOA is forced to be the NRCA. As shown, the NRCAis illustrated by the segment 382.

Display System

[0174] A display system, generally identified with the reference numeral400, is illustrated in FIGS. 23-42. The display system 400 is used toprovide a visual indication of the terrain advisory and terrain warningindications discussed above as a function of the current position of theaircraft. Background terrain information is also provided which providesan indication of significant terrain relative to the current position ofthe aircraft.

[0175] In order to declutter the display, background information may bedisplayed in terms of predetermined dot patterns whose density varies asa function of the elevation of the terrain relative to the altitude ofthe aircraft. Terrain advisory and terrain warning indications may bedisplayed in solid colors, such as yellow and red, relative to thegroundtrack of the aircraft.

[0176] An important aspect of the invention is that the terrainbackground information as well as the terrain advisory and warningindications may be displayed on a navigational or weather display,normally existing within an aircraft, which reduces the cost of thesystem and obviates the need for extensive modifications to existingdisplays, such as a navigational and weather radar type display. Inparticular, the terrain data is converted to a weather radar format inaccordance with the standard ARINC 708/453 for digital buses on civilaircraft. The terrain data, masked as weather data, can then be easilydisplayed on an existing navigational or a dedicated weather radardisplay that conforms with the ARINC 708/453 serial interface standard.

[0177] Referring to FIG. 23, the display system 400 includes a weatherradar display or navigational display 36 connected to a control bus 404conforming to an ARINC 429 standard and a terrain data bus switch 406.The terrain data bus switch 406 enables selection between terrain dataand weather data for display. More particularly, the data bus switch 406includes a common pole 408 connected to the display 36 by way of aserial display bus 410. One contact 412 on the data bus switch 406 isconnected to a serial weather data bus 414 and, in turn, to a weatherradar R/T unit 416, which transmits weather radar data over the weatherdata bus 414 in conformance with the ARINC 708/453 standard.

[0178] An antenna 418 provides weather radar data to the weather radarR/T unit 416. The weather radar R/T unit 416 is also connected to thecontrol bus 404, which is used for range and mode data.

[0179] In accordance with an important aspect of the invention, theterrain advisory and terrain warning indications, as well as thebackground information discussed above, are converted to “RHO/THETA”format. The converted terrain data is then applied to a terrain dataserial bus 420, which conforms to the ARINC 453 standard, and connectedto a contact 422 on the data bus switch 406 to enable selective displayof either weather radar or terrain data on the display 36.

[0180] The system for converting the terrain advisory and warningindications as well as background terrain data to an ARINC 708/453standard serial format is indicated as a functional block 424 in FIG. 23and is described in more detail below. By converting such data into anARINC 708/453 standard serial format, the data can be displayed on anexisting display 36, which conforms to the ARINC 453 standard withoutthe need for relatively complex and expensive modifications to thedisplay's symbol generator.

[0181] Referring to FIG. 24, each word 425, in accordance with the ARINC453/708 standard, is 1600 bits long and represents one spoke or radial426 of the display 36. Normally, 512 spokes or radials 426 aretransmitted for each complete sweep of the display 36. At a transmissionrate of, for example, 1 megahertz (MHz), each word 425 takes about 1.6milliseconds (msec) to transmit. For 512 spokes, one complete sweep willthus take about 4 seconds.

[0182] The 1600-bit words 425 include a header 428 and 512 range bins430. The header 428 includes the control data on the control bus 404 andalso includes a 12-bit code representing the antenna scan angle; theangle of the spoke relative to the aircraft heading equivalent to anindicator UP direction. The range bins 430 cover the distance from theantenna 418 to the maximum selected. Each of the 512 range bins includesthree intensity bits, encoded into colors as indicated in TABLE 3 below.TABLE 3 3-BIT RANGE CODE COLOR 000 BLACK 100 GREEN 010 YELLOW 110 RED001 — 101 CYAN 011 MAGENTA 111 —

[0183] As will be discussed in more detail below, the display system 400is capable of displaying background terrain information as well asterrain threat indications as a function of the current position of theaircraft. The threat detection algorithm is run along the groundtrack ofthe aircraft. Thus, terrain threat indications are typically displayedalong the groundtrack while background terrain is displayed relative tothe heading of the aircraft.

[0184] Referring to FIG. 25, the terrain background information is shownon the display 36. As will be discussed in more detail below, theelevation of the highest terrain relative to the altitude of theaircraft is shown as a series of dot patterns whose density varies as afunction of the distance between the aircraft and the terrain. Forexample, a relatively dense dot pattern 432 may be used to indicateterrain that is, for example, 500 feet or less below the aircraft. Amedium dense dot pattern 434 may be used to represent terrain that is1000 feet or less below the aircraft, while a lightly dotted pattern 436may be used to indicate terrain 2000 feet or less below the aircraft. Inorder to declutter the display 402, terrain more than, for example, 2000feet below the aircraft, is not shown. The dots may be displayed in oneof the colors indicated in TABLE 3 above, for example, yellow (amber).In addition, in the vicinity of an airport, a runway graph 438, forexample, in green, may also be provided. The apparent display resolutionmay be increased by using more than one color along with the variabledensity dot patterns, for example, as shown in TABLE 4 below. TABLE 4DOT DENSITY PATTERN HIGH 3000   0 MEDIUM 4000 1000 LOW 5000 2000 COLORGREEN YELLOW

[0185] The display of a terrain threat indication is illustrated in FIG.26. As shown, the terrain threat indication may be displayedcontemporaneously with the background terrain information. Terrainadvisory and terrain warning indications are displayed in solid shapes440 and 442, respectively, for example, “squares”; the displayed terrainmap cells which represent a threat painted solid yellow or red. Moreparticularly, at an aspect ratio for the display of, for example, 3×4(vertical to horizontal), the terrain cells will appear “square”,particularly at relatively low-range settings. Colors are used todistinguish between terrain advisory and terrain warning indications.For example, red may be used to represent a terrain warning indication442 while yellow or amber is used to represent a terrain advisoryindication 440. By using colored shapes for terrain threat indicationsand dot patterns of variable density for the terrain backgroundinformation, clutter of the display 402 is minimized.

[0186] The terrain data from the terrain data base 24, for example, asillustrated in FIG. 4B, is used to compute the terrain threatindications as well as background information for the display 36. Asdiscussed above, the terrain data base 24 may be stored in a flash ROM444 (FIG. 27) for convenience of updates. In order to facilitate updateof the display 36 as a function of the position of the aircraft, terraindata from the flash ROM 444 is transferred to a random access memory(RAM) A, identified with the reference numeral 446. As will be discussedin more detail below, the RAM A is mapped with terrain data from theflash ROM 444, centered about the current position of the aircraft, asillustrated in FIG. 27.

[0187] In order to provide a relatively large display range, forexample, 160 nautical miles, while minimizing processing time as well asthe size of the RAM A, the RAM A is configured as a “wedding cake” asgenerally illustrated in FIG. 28 and configured such that the highestterrain resolution is near the position of the aircraft and the terrainfarthest from the aircraft position has the lowest resolution. Forexample, the RAM A may be provided with four (4) layers. As shown, theinstantaneous aircraft position is shown generally centered in layer 1,having the highest resolution. The resolution of layer 2 lies betweenthe resolution of layer 3 and layer 1. Layer 4 (not shown) is providedwith a relatively coarse resolution for use in a cruise mode.

[0188] Such a variable resolution display, configured as a layeredwedding cake, minimizes the memory size of RAM A as well as theprocessing time. In order to improve the performance of the system, theflash ROM 444 may also be configured similar to a “wedding cake”, asgenerally illustrated in FIG. 27.

[0189] As discussed above, the terrain data base 24 may be provided withvariable resolution with a relatively fine resolution of, for example,30×20 arc sec (i.e. ½×½ arc min). The relatively coarse resolution oflayers 2 and 3 can be formed by combining finer resolution maps. Withsuch a resolution, the configuration of RAM A may be selected inaccordance with TABLE 5. TABLE 5 RESOLUTION LAYER SIZE LAYER (MINUTES)(MINUTES) 1 ½ × ½ 32 × 32 2 1 × 1 64 × 64 3 2 × 2 128 × 128 4 5 × 5REMAINING AREA

[0190] Each layer is generally centered relative to the aircraftposition as shown in FIG. 28 at the time of the update of RAM A. Eachlayer of the “wedding cake” has its own update boundary. For example,layer 1 of the RAM A includes an update boundary 448. As the aircraftpasses through the update boundary 448, the RAM A is updated with newterrain data from the flash ROM 444 (FIG. 27) as will be discussed inmore detail below.

[0191] Because of the refresh rates required for the display 36, theterrain data may be stored in files alternative to the formatillustrated in FIG. 4A. For example, the file format of the terrain datamay include an index file or header 55, stored in one or more 128 K bytestorage blocks and a plurality of data files 51, as generally shown inFIG. 29.

[0192] As shown, each index file 55 may include pointers to specificdata files 51, the length of the map file, as well as the position ofone or more of the corner boundaries of the file 51, for example,northwest and southeast corners of each map file 51, which facilitatesupdates of the display 36 as a function of the current aircraftposition. The index file 55 may contain information regarding the lengthof the map file 51, a start of file or altitude offset, the resolutionof the map file 51, and whether and how the file 51 is compressed toenable use of various compression algorithms.

[0193] The data files 51 may include a file header 454 as well as datablocks 456 which, as shown, can be grouped according to resolution size.The file header 454 indicates various information about the file. Theformat of the header 454 may be structured as follows:

[0194] <MAJOR VERSION BYTE><MINOR VERSION BYTE><FILE STATUS BYTE><ATTRIBBYTE>

[0195] <FILE NAME (8 CHARACTERS)><EXTENSION (4 CHARACTERS)>

[0196] <FILE LENGTH LONGWORD>

[0197] <TIME STAMP LONGWORD>

[0198] <CRC LONGWORD>

[0199] <SPME LONGWORD>

[0200] The major and minor version bytes relate to the revision statusof the data in the data files 51. The minor version is set as theinverse of the byte value of the major version such that a default erasewill indicate a zero version. The file status byte 457 provides variousinformation regarding the status of the file as discussed below, whilethe attribute byte is reserved. The file name, extension file length,and spare data are self-explanatory. The time stamp relates to theoperational time of the system (i.e. length of time in use) while theCRC data relates to the data files 51.

[0201] The status byte may be used to provide the following informationregarding the data blocks 456, for example, as set forth in TABLE 6below. TABLE 6 FLAG BYTE VALUE INFORMATION FF EMPTY FE DOWNLOADING FOVALID EO READY TO ERASE . RESERVED . RESERVED 00 RESERVED

[0202] The EMPTY status (FF) is set by erasing the data blocks 456,while the DOWNLOADING status (FE) is set as part of the image. The VALIDstatus (FO) is set when the file is loaded. Lastly, the READY TO ERASE(EO) is set by the file system for obsolete blocks. The file structuremay also be as described in Appendix 1.

[0203] As indicated below, various corrections of the data files 51 maybe required in areas near runways. FIG. 30 illustrates a portion of theterrain data base 24 around a particular airport, Boeing Field, with aresolution of, for example, ½×½ minute subcells 455, shown with a runway456 drawn in. The elevations, as discussed above, are selected to be thehighest elevations within the cells 455, rounded up to the next highestresolution increment, for example 100 feet. While such a system providesuseful and conservative terrain data for use with the terrain data base24, such a system in the vicinity of an airport can cause the elevationsof the runway 456 to be too high or too low. For example, the elevationof the runway 456 at Boeing Field is known to be 17/15 feet; rounded offto zero (0) feet. FIG. 30 shows the elevation of runway 456 to be200/300 feet, for example, by the averaging algorithm discussed above.Such errors in the runway elevation can degrade system performance, and,in particular, cause improper performance of the terrain advisory andterrain warning cut-off boundaries below 500 feet, for example, asillustrated in FIG. 22.

[0204] In order to solve this problem, the actual elevation of therunway 456, rounded to the nearest resolution, is used for cells 455around a perimeter of the runway 456, as generally shown in FIG. 31. Inparticular, a first perimeter, illustrated by the dashed box 458, isselected to be about ±1 cell (i.e. ½ minute) from the centerline of therunway 456. The elevations in all of the cells 455 intersected by thedashed box 458 are set to the actual runway elevation and rounded to thenearest resolution. For Boeing Field, the elevation for such cells is 0feet.

[0205] Consideration is also given for a narrow approach and a wideapproach. The dashed box 460 represents a wide approach while the dashedbox 462 represents a narrow approach. The cells 464 and 466, intersectedby the wide approach box 460, are set on the actual runway elevation,rounded to 0 feet. Inside the approach perimeter, as indicated by thearc 467, the elevation for the cells is selected to be either the runwayelevation or the ΔH terrain floor (as discussed above). For example, ifthe corner of the cell 468, closest to the runway 456, is 1.386 nM fromthe runway 456, the ΔH terrain floor will be 86 feet; rounded to nearest100 feet=100 feet. Thus, the elevation of cell 468 is selected as 100feet.

[0206]FIG. 32 illustrates the differences between the elevations inselected cells 468, 470 and 472 before and after correction. Beforecorrection, the elevation of cells 468, 470 and 472 was 300 feet. Aftercorrection, the elevation of cells 470 and 472 are corrected to therunway elevation and rounded to the nearest resolution (i.e. 0 feet).The cell 468 is corrected to the ΔH terrain floor elevation of 100 feet.

[0207]FIG. 33 illustrates another situation where the elevation ofcells, due to the rounding-off method discussed above, can result in asituation where the cell elevations adjacent a runway, represented as astaircase 474, are below the runway elevation. The correction method,discussed above, can also be used to correct the various cells aroundthe runway 456 up to the runway elevation in a manner as generallydiscussed above.

[0208]FIG. 34 represents a simplified block diagram for implementationfor the display system 400 in accordance with the present invention. Thedisplay system 400 may include a microprocessor, for example, an Inteltype 80486, 25 MHz type microprocessor (“486”) and an Analog Devicestype digital signal processor (DSP). The DSP is primarily used forcalculating the RHO/THETA conversions to off load the 486.

[0209] The display system 400 may have a normal mode and a cruise mode.In a normal mode, the display system 400 displays terrain out to, forexample, 100 nautical miles, while in cruise mode out to about 320nautical miles. In the cruise mode, it is contemplated that thebackground terrain display levels can be lowered by the pilot such thatterrain, for example 10,000 feet or 20,000 feet below the aircraft, canbe displayed. Alternatively, the display level could automatically beadjusted to the highest terrain within the selected display range; thevalue of this level set forth in amber, for example.

[0210] The 486 runs a “file picker” routine which searches the terraindata base 24 for terrain data covering the area relative to the currentposition of the aircraft. For example, in a normal mode, the file pickerroutine searches the terrain data base 24 for terrain data covering anarea approximately 256×256 minutes relative to the current position ofthe aircraft. As indicated above, the terrain data base 24 includesindex files 55 (FIGS. 4B and 29) which may include the northwest andsoutheast corners of each map file 51 in order to facilitate the search.A list of files covering the area is assembled and maintained.

[0211] As indicated above, the index file 55 also includes informationas to whether and how the data files 51 are compressed. The files may becompressed/decompressed by a modified Hoffman code, such as PKZIP asmanufactured by PKWARE, INC.

[0212] If the data files 51 are compressed, the files are decompressedin the background because of the relatively long decompression time andstored in a decompressed file RAM (DFR) 476, for example 256 K bytes.The decompressed files are then transferred to RAM A, identified withthe reference number 446. On power-up, one or two files around theaircraft are decompressed and displayed to allow immediate display of alimited range, relative to the position of the aircraft. As more terraindata files are decompressed, the additional range is displayed.

[0213] The RAM A may be configured in a plurality of cells 479 (FIG.34), representative of a terrain area ½×½ minutes, with true northappearing at the top of RAM A as shown. Each cell 479 is two bytes wideand includes data regarding the highest altitude in the cell as well asthe identity of each cell 479. As such, the RAM A is addressable by Xand Y coordinates and thus represents a map with an altitude in eachcell 479. The identity of the cell may be used to distinguish betweenterrain data, for example, from the terrain data base 24 and obstacledata. As discussed above, in some applications, an obstacle data basemay be utilized.

[0214] As indicated above, the RAM A may be configured as a wedding cakewith various layers, as generally shown in FIG. 28 with the top layer(i.e. layer 1) having the highest resolution (i.e. ½×½ minute cells).Terrain data is loaded into the RAM A such that the aircraft is roughlycentered in the top layer, where the terrain advisory and warningdetection algorithms are run. Once the aircraft moves outside the updateboundary 448 (FIG. 28), the top layer of the RAM A is reloaded withterrain data with the aircraft centered. The lower layers may also beprovided with update boundaries, which enable the lower layers to beupdated independently and less frequently than the top layer.

[0215] As discussed above and illustrated in FIG. 27, the flash ROM 444for the terrain data base 24 may also be configured as a wedding cake asillustrated in FIG. 28. In such an implementation, the highestresolution is provided near the airports while the coarser resolution isprovided away from the airports as generally shown in FIG. 27. Forexample, as illustrated in FIG. 34A, the terrain data base 24 may beconfigured with 1°×1°, 2°×2° or 10°×10° map files. Some of the map filesmay overlap, similar to a “wedding cake”. As shown, the terrain mapsclosest to airports are provided with the highest resolution, forexample 15×15 arc seconds. Maps further away from the airport may beprovided with resolutions of, for example, 1×1 minute or 2×2 minuteswhile a coarse resolution (i.e. 2°×2° or 10°×10°) may be provided evenfurther from the airport.

[0216] An upload routine may be used to upsample or downsample terrainor obstacle data into the RAM A from the DFR RAM 476 as shown in FIG.34. Such a routine is well within the ordinary skill in the art. The topthree layers (i.e. layers 1, 2 and 3) of the RAM A (FIG. 28) are updatedfrom the map file source from the flash ROM 444 for the terrain database 24 having the highest resolution, except for the 5×5 minute layer,which is updated only from a 5×5 minute file source to reduce processingtime.

[0217] Alternatively, as shown in FIG. 34A, an additional RAM A*,identified with the reference numeral 477, may be provided andstructured similar to the RAM A 446, forming a “ping-pong” memoryarrangement. In this embodiment, one terrain data file 51 isdecompressed one file at a time and loaded into the DFR RAM 476 and thenupsampled to either the RAM A or RAM A*. For example, while the RAM A isbeing loaded with additional decompressed files, the terrain threat anddisplay algorithms are executed from the RAM A*. Once the RAM A isupdated, the RAM A is then used for the computations while the other RAMA* is updated with new data from the DFR RAM 476 to account for changesin the aircraft position.

[0218] The 5×5 minute resolution layer is primarily intended for use ina cruise mode to provide a look-ahead range of about 320 nM. Asdiscussed above and illustrated in FIG. 29, each file includes an indexfile 55 which contains data relative to the resolution of the terraindata in a particular map file 51. Such information can be used to varythe resolution of the map files as a function of longitude and latitudeposition. For example, the latitude resolutions of the map files can bedecreased at higher latitudes in a northward direction. Moreover,longitude resolution in the RAM A can be varied and provided with, forexample, a one degree latitude hysteresis to prevent flipping back andforth as an aircraft cruises northwardly. Once the terrain data from theterrain data base 24 is loaded into the RAM A, the terrain advisory andwarning algorithms (“detection algorithms”) are run once per second asdiscussed below.

[0219] The terrain threat algorithms discussed above are based on asingle vector along the groundtrack. The single vector providesacceptable results because the relatively crude resolution of theterrain data base 24 is based upon the highest elevation per cell andthe natural noisiness of the groundtrack angle and position information.However, the single vector approach does not take into account variouserrors, due to, for example, the terrain data base 24, GPS longitude andlatitude errors as well as the published track angle accuracy. As such,the single vector along the groundtrack may be substituted with an arrayof vectors, as generally shown in FIG. 35, which takes such errors intoaccount. In particular, the groundtrack vector is identified with thereference numeral 482 originating at the instantaneous aircraft positionX₀, Y₀. A pair of vectors 484 and 486 are disposed at a configurabledistance D_(OFF), for example 0.2 nM along a segment 488, perpendicularto the vector 482 at points X₁, Y₁, and X_(r), Y_(r). A pair of outsidevectors 490 and 492, originating at points X₁, Y₁ and X_(r), Y_(r),respectively, are directed at a configurable angle D_(ALPHA), relativeto the vectors 484 and 486. The configurable angle D_(ALPHA) may beselected as 3 degrees, equivalent to the groundtrack accuracy of theGPS.

[0220] The coordinates X₁, Y₁ and X_(r), Y_(r), for the vectors 490 and492, respectively, are determined. More particularly, the incremental Xand Y vectors, DX_(OFF) and DY_(OFF), are determined as set forth inequations (25) and (26) as set forth below.

DX _(OFF) ={D _(OFF) /COS(LAT)}*COS (GROUNDTRACK),  (25)

[0221] where D_(OFF) is a configurable distance between X₀, Y₀ and X₁,Y₁, for example 0.2 nm and the factor 1/COS(LAT) is used to convertDX_(OFF) to nautical miles when DX_(OFF), DY_(OFF) is in minutes andD_(OFF) is in nautical miles.

DY _(OFF) =D _(OFF) *SIN(GROUNDTRACK)  (26)

[0222] The coordinates X_(r), Y_(r) can be determined in a similarmanner.

[0223] The threat detection algorithms are initiated by calculating therange increments DELTA X_(NS) and DELTA Y_(NS) along a north-southcoordinate system along the groundtrack. Referring to FIG. 36, eachspoke 426 has 512 size range bins as discussed above, or 256 doublerange bins. In order to convert from an X-Y coordinate system to thenorth-south coordinate system, the range increments DELTA X_(NS), DELTAY_(NS) are determined in accordance with equations (27) and (28). (Theterm 1/COS (LATITUDE) is used to convert from arc minutes to nauticalmiles.)

DELTA X _(NS)=DETECTION STEP SIZE*SIN(GROUNDTRACK)*1/COS(LATITUDE)  (27)

DELTA Y _(NS)=DETECTION STEP SIZE*COS (GROUNDTRACK)  (28)

[0224] After the range increments DELTA X_(NS) and DELTA Y_(NS) aredetermined, the threat detection algorithm computation is initiated atthe instantaneous position X₀, Y₀ of the aircraft (FIG. 37). Theposition is then incremented by the range increments DELTA X_(NS) andDELTA Y_(NS) and the range is incremented by a configurable step size,for example ⅛ unit, as discussed above. After the new coordinates aredetermined, the current terrain altitude at the new coordinate isdetermined. The terrain detection algorithms, as discussed above, arethen computed for the current advisory LAD, as indicated by the arc 494,defining a yellow threat detection vector 496. In order to provideadditional warning time for a terrain warning, the terrain warningindication can be computed out to a distance equal to 1.5*LAD, asindicated by the arc 498 to define a red threat detection vector 500.The above steps are repeated for the look-ahead vector arrays 490 and492 (FIG. 35). The entire computation is done at a rate of once persecond.

[0225] Whenever the threat detection algorithm detects terrain along thelook-ahead vector arrays 490 and 492 (FIG. 35), the offending terrain ispainted in an expanded cone around the groundtrack as shown in FIG. 37.In particular, the expanded cone includes a plurality of vectors,spaced, for example, 4° apart (configurable value), for ±90°(configurable value) on each side of the groundtrack. For an expandedterrain warning, the cone may be expanded out distances, for example asset forth in TABLE 7 below. TABLE 7 (For an expansion factor of 2)NORMAL EXPANDED LOOK- LOOK- TYPE OF AHEAD/ AHEAD/ THREAT LOOK-DOWNLOOK-UP LOOK-DOWN LOOK-UP ADVISORY 1.0 * 2.0 * 2.0 * 4.0 * LAD LAD LADLAD WARNING 0.5 * 1.5 * 1.0 * 3.0 * LAD LAD LAD LAD

[0226] The terrain threat is painted on the display 36 by calculatingthe starting vector angle DISPA as set forth in equation (29) when aterrain threat along the groundtrack has been detected.

DISPA=GROUNDTRACK+30 degrees  (29)

[0227] The range increments are then determined as set forth inequations (30) and (31).

DELTA X _(NS)=DISPLAY STEP SIZE*SIN(DISPA)*1/COS(LATITUDE)  (30)

DELTA Y _(NS)=DISPLAY STEP SIZE*COS(DISPA),  (31)

[0228] where the display step size is a configurable constant, forexample ½ unit.

[0229] After the range increments DELTA X_(NS) and DELTA Y_(NS) aredetermined, the instantaneous position of the aircraft X₀, Y₀ isincremented by the same and the range is incremented by the step size.The altitude is then determined for the current position on the terrainmap. Subsequently, the threat detection algorithms are run for thedistances as discussed above. Should a threat be detected, a color isloaded into a RAM B, identified with the reference numeral 504 (FIG.34). At the end of the yellow and red threat vectors 496 and 500, thevector angle DISPA is incremented by 4 degrees with the above stepsbeing repeated until all vectors are done.

[0230] In addition to the terrain detection and display algorithms, abackground terrain algorithm is also executed relative to the terraindata in RAM A (FIG. 34) once every screen update, about 4 seconds. Thebackground terrain algorithm simply compares the current altitude of theaircraft with the various terrain altitude data in the RAM A. Any of thedot patterns 432, 434 and 436 (FIG. 25) resulting from the computationare stored in the RAM B (FIG. 34) organized in a wedding-cakeconfiguration (FIG. 28), similar to the RAM A (FIG. 34). In addition,the RAM B is organized in single byte cells containing the color/dotpattern as discussed above. The closest runway may also be stored in theRAM B as either a circular or square pattern in, for example, a greencolor.

[0231] The color/dot pattern in the RAM B (FIG. 34) is transferred tothe DSP memory RAM B*, identified with the reference numeral 506, onceper screen update (i.e. 4 sec) and synchronized to the DSP screenpainting period. If a threat is detected as discussed above, a new RAM Bimage is transferred to the DSP RAM B* 206 immediately. The imagetransfer time typically fits between two spokes. If not, the DSP willsend an old spoke stored in its spoke memory 508, 510 twice.

[0232] The DSP then performs the RHO/THETA conversion on the data in RAMB* 506 using the updated aircraft position, selected range and headingdata from the 486, as discussed below. Changes in the selected range areimplemented on the fly (i.e. the next spoke) without restarting thesweep.

[0233] As discussed above, the ARINC 708/453 weather radar formatconsists of 512 spokes covering a configurable scan range of typically±90 degrees and a range selected, for example, on a weather data controlpanel. For ARINC systems, each spoke is calculated twice, once for theleft selected range and once for the right selected range. The spokesare then transmitted by way of a serial port 512, interleaved left spokeand right spoke to the bus switch 406 (FIG. 23) by way of a pair ofdrivers 514 and 516 (FIG. 34).

[0234] The resolution of the scan conversion is 256 range bins; eachrange bin actually being a double range bin per the ARINC 708/453standard format. Such a resolution is sufficient for the dot patterns518 illustrated in FIGS. 25 and 26.

[0235] The RHO/THETA conversion is best understood with reference toFIG. 38. In particular, once per sweep (i.e. every 4 seconds) the rangeincrement R1 is determined. The range increment is the selected rangedivided by 256. After the range increment is determined, the startingspoke angle is determined (i.e., the aircraft true heading plus 90degrees). Lastly, the incremental spoke angle DELTA ALPHA is determined.The incremental spoke angle DELTA ALPHA is the sweep range divided by512; the total number of spokes per sweep.

[0236] Once per spoke (i.e. 512 times per 4 second sweep), theincremental range coordinates DELTA X_(ns), DELTA Y_(ns) are computedand the spoke angle is incremented by DELTA ALPHA, as discussed above.The incremental range increment coordinates X_(ns) and Y_(ns) arecomputed in accordance with equations (32) and (33), respectively.

DELTA X _(ns) =RI*SIN(SPOKE ANGLE)*1/COS(LATITUDE)  (32)

DELTA Y _(ns) =RI*COS(SPOKE ANGLE)  (33)

[0237] Once per range increment (i.e. 512×256 times per 4 second sweep),the instantaneous aircraft position X₀, Y₀ is incremented by DELTAX_(ns) and DELTA Y_(ns). The color/pattern for the new position islooked up and output to the display 36. These steps are repeated for therest of the spoke.

[0238] With the use of the DSP, two computations can be done inparallel. In particular, the color/pattern of each range bin includingthe conversion of the longitude to nautical miles (i.e. multiplicationby 1/COS (LAT)). With reference to FIG. 38, this computation isperformed relative to the true north map coordinate system with theaircraft true heading pointing up and the spoke angles referenced to it.The second computation addresses each display indicator pixel in orderto paint the required dot pattern, relative to the display screencoordinate system.

[0239] The generation of dot pattern is illustrated in FIGS. 39-42. Ingeneral, the scan conversion is simulated by overlaying each RHO/THETArange bin over a calculated indicator pixel map which includescorrections for display aspect ratio using fractal 8×8 dot patterns.

[0240] The fractal patterns corresponding to the variable density dotpatterns 432, 434 and 436 (FIG. 25) are illustrated in FIG. 40. Thefractal pattern size is selected to be 8×8 pixels. The densest pattern432 includes ½, or 32, of the 64 pixels filled. In the medium densitypattern 434, 16, or ¼, of the pixels are filled. In the least densepattern 4 of 64, or {fraction (1/16)}, of the pixels are filled.

[0241] Referring to FIG. 39, the display screen is divided into a pixelmap, consisting of, for example, 256×256 pixels 520. As mentioned above,the 8×8 fractal patterns are overlaid onto the pixel map to provide thevariable density dot patterns 432, 434 and 436 illustrated in FIG. 25.

[0242] The computation of the dot patterns is best understood withreference to FIGS. 39, 41 and 42. Using an X-Y coordinate system, eachspoke is determined at the bottom center of the display 36. For a256×256 pixel display, the starting point corresponds to 120, 0. Next,the incremental X and Y coordinates relative to the scan angle ALPHA aredetermined according to equations (34) and (35).

DELTA X=RI*SIN(ALPHA)  (34)

DELTA Y=RI*COS(ALPHA),  (35)

[0243] where RI=a double range increment and ALPHA=the scan angle. Theseincrements are added to the starting point 128, 0. After each increment,the selected fractal pattern is looked up in the RAM B * to determinewhether to send the last range increment as either block or colored.

[0244] Referring to FIG. 42, the increments DELTA X and DELTA Y areapplied to a pair of adders 522 and 524, which, in turn, are applied toan X-counter 526 and a Y-counter 528. As discussed above, the X counter526 starts at 128 and counts up if DELTA X>0. If DELTA X<0, theX-counter counts down, indicating a pixel on the left (FIG. 39) of thestarting position. Since the starting pixel is at the bottom of thedisplay, the Y-counter 528 starts at zero and always counts up. A rangecounter 528 counts up to 256; the maximum number of double range bins.The range counter 528 is driven by an increment clock such that thepattern/color is painted once per range increment. The output indicateswhether the pixel is to be painted left black.

[0245] As discussed above, the background terrain algorithm determinesthe particular fractal pattern to be used. This data is applied to theDSP to enable the pixels on the pixel map (FIG. 29) to be painted orleft blank. In particular, the three least significant bits from the Xcounter 526 and Y counter 528 are used to select the fractal pattern asshown.

[0246] The scan conversion simulation is performed in parallel with theRHO/THETA conversion which determines the color/dot pattern of the rangebin to be transmitted. The color/dot pattern provided by the RHO/THETAconversion is then overlaid with the fractal dot pattern to determinewhether the range bin color should be black or painted to provide theproper dot pattern on the display.

[0247] Obviously, many modifications and variations of the presentinvention are possible in light of the above teachings. Thus, it is tobe understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedabove.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A warning system for an aircraft comprising: firstmeans for receiving signals representative of the instantaneous flightpath angle; second means for receiving signals from a predeterminedsatellite-based navigation system representative of the position of theaircraft; third means for receiving signals representative of thealtitude of an aircraft; means for storing data representative ofterrain elevation information; and means responsive to said first means,said second means, said third means and said storing means forgenerating a first warning signal of a hazardous flight condition, as afunction of both the position of the aircraft relative to said terrainelevation information and the flight path angle of aircraft, for apredefined area relative to the aircraft wherein said area includesareas both above and ahead of the aircraft.
 2. A warning system asrecited in claim 1, wherein said predetermined satellite-basednavigation system is the global positioning system GPS.
 3. A warningsystem for an aircraft comprising: means for receiving signalsrepresentative of the instantaneous flight path angle of the aircraft;means for receiving signals representative of the position of theaircraft; means for receiving signals representative of the altitude ofthe aircraft; means for storing data representative of actual terrainelevation information; and means for generating a first warning signalof a hazardous flight condition resulting from location of a subset ofsaid predetermined terrain information at a predetermined distance infront of the aircraft, said first warning signal generated as a functionof both the position of the aircraft relative to said subset of saidpredetermined terrain information and the flight path angle of theaircraft.
 4. A warning system as recited in claim 23, wherein saidgenerating means includes means for determining the relationship betweenthe flight path angle and a predetermined first datum and generating afirst warning signal when a predetermined relationship between theflight path angle and said first predetermined datum exists.
 5. Awarning system as recited in claim 24, wherein said generating meansincludes means for determining the relationship between the flight pathangle and a predetermined second datum and generating a second warningsignal when a predetermined relationship between said flight path angleand said second predetermined datum exists.
 6. A warning system asrecited in claim 25, wherein parameters for determining said firstpredetermined datum and said second predetermined datum areconfigurable.
 7. A warning system as recited in claim 23, wherein saidpredetermined distance is a function of the airspeed of the aircraft. 8.A warning system as recited in claim 23, wherein said first warningsignal defines a plurality of boundaries relative to the currentposition and altitude of the aircraft.
 9. A warning system as recited inclaim 28, further including means for modifying said boundaries underpredetermined conditions.
 10. A warning system as recited in claim 29,wherein said modifying means includes means for modifying saidboundaries as a function of the flight path angle of the aircraft. 11.The warning system of claim 1 wherein said area relative to the aircraftfurther includes an area below the aircraft.
 12. A method of providingterrain warnings for a vehicle, the method comprising: receiving signalsrepresentative of an instantaneous flight path angle of the vehicle;receiving signals representative of the position of the vehicle;receiving signals representative of terrain elevation information; andgenerating a first warning signal as a function of both the position ofthe vehicle relative to the terrain elevation information and the flightpath angle of the vehicle for a predefined area relative to the vehiclewherein the area includes areas both above and ahead of the vehicle. 13.A method of providing terrain warnings for a vehicle, the methodcomprising: receiving signals representative of an instantaneous flightpath angle of the vehicle; receiving signals representative of theposition of the vehicle; receiving signals representative of thealtitude of the vehicle; receiving signals representative of terrainelevation information; and generating a first warning signal as afunction of a subset of the terrain elevation information at apredetermined distance in front of the aircraft, said first warningsignal generated as a function of both the position of the vehiclerelative to the subset of terrain elevation information and the flightpath angle of the vehicle.